Modified acquisition strategy for reduced motion artifact in super resolution T2 FSE multislice MRI: Application to prostate.

PURPOSE: To reduce slice-to-slice motion effects in multislice T2 -weighted fast-spin-echo ( T2 FSE) imaging, manifest as "scalloping" in reformats, by modification of the acquisition strategy and to show applicability in prostate MRI. METHODS: T2 FSE images of contiguous or overlapping slices are typically acquired using multiple passes in which each pass is comprised of multiple slices with slice-to-slice gaps. Combination of slices from all passes provides the desired sampling. For enhancement of through-plane resolution with super resolution or for reformatting into other orientations, subtle ≈1 mm motion between passes can cause objectionable "scalloping" artifact. Here we address this by subdivision of each pass into multiple segments. Interleaving of segments from the multiple passes causes all slices to be acquired over substantially the same time, reducing pass-to-pass motion effects. This was implemented in acquiring 78 overlapped T2 FSE axial slices and studied in phantoms and in 14 prostate MRI patients. Super-resolution axial images and sagittal reformats from the original and new segmented acquisitions were evaluated by 3 uroradiologists. RESULTS: For all criteria of sagittal reformats, the segmented acquisition was statistically superior to the original. For all sharpness criteria of axial images, although the trend preferred the original acquisition, the difference was not significant. For artifact in axial images, the segmented acquisition was significantly superior. CONCLUSIONS: For prostate MRI the new segmented acquisition significantly reduces the scalloping motion artifact that can be present in reformats due to long time lags between the acquisition of adjacent or overlapped slices while retaining image sharpness in the acquired axial slices.

Use of kZ -space for high through-plane resolution in multislice MRI: Application to prostate.

PURPOSE: The goal of this work is to demonstrate 1 mm through-plane resolution in multislice T2SE MRI using k Z -space processing of overlapping slices and show applicability in prostate MRI. METHODS: Multiple overlapped slices are acquired and Fourier transformed in the slice-select direction. The slice profile is incorporated into a Tikhonov-regularized reconstruction. Through-plane resolution is tested in a resolution phantom. An anthropomorphic prostate phantom is used to study the SNR, and results are compared with theoretical prediction. The proposed method is tested in 16 patients indicated for clinical prostate MRI who gave written informed consent as overseen by our IRB. The "proposed" vs. "reference" multislice images are compared using multiple evaluation criteria for through-plane resolution. RESULTS: The modulation transfer function (MTF) plots of the resolution phantom show good modulation at frequency 0.5 lp/mm, demonstrating 1 mm through-plane resolution restoration. The SNR measurements experimentally match the theoretically predicted values. The radiological evaluation shows that the proposed method is superior to the reference method for five criteria of sharpness but inferior with respect to artifacts. CONCLUSIONS: In conjunction with overlapped slices a k Z -space-based reconstruction approach can be used to improve through-plane resolution in multislice T2SE MRI. 1 mm resolution is demonstrated from 3.2 mm thick slices. The in vivo results from prostate MRI show improved sharpness when compared to the standard multislice method.

Time-resolved contrast-enhanced MR angiography with single-echo Dixon fat suppression.

PURPOSE: Dixon-based fat suppression has recently gained interest for dynamic contrast-enhanced MRI, but multi-echo techniques require longer scan times and reduce temporal resolution compared to single-echo alternatives without fat suppression. The purpose of this work is to demonstrate accelerated single-echo Dixon imaging with high spatial and temporal resolution. THEORY AND METHODS: Real-valued water and fat images can be obtained from a single measurement if the shared initial phase and that due to ΔB0 are assumed known a priori. An expression for simultaneous sensitivity encoding (SENSE) unfolding and fat-water separation is derived for the general undersampling case, and simplified under the special case of uniform Cartesian undersampling. In vivo experiments were performed in extremities and brain with SENSE acceleration factors of up to R = 8. RESULTS: Single-echo Dixon reconstruction of highly undersampled data was successfully demonstrated. Dynamic contrast-enhanced water and fat images provided high spatial and temporal resolution dynamic images with image update times shorter than previous single-echo Dixon work. CONCLUSION: Time-resolved contrast-enhanced MRI with single-echo Dixon fat suppression shows high image quality, improved vessel delineation, and reduced sensitivity to motion when compared to time-subtraction methods.

Robust and efficient pharmacokinetic parameter non-linear least squares estimation for dynamic contrast enhanced MRI of the prostate.

PURPOSE: To describe an efficient numerical optimization technique using non-linear least squares to estimate perfusion parameters for the Tofts and extended Tofts models from dynamic contrast enhanced (DCE) MRI data and apply the technique to prostate cancer. METHODS: Parameters were estimated by fitting the two Tofts-based perfusion models to the acquired data via non-linear least squares. We apply Variable Projection (VP) to convert the fitting problem from a multi-dimensional to a one-dimensional line search to improve computational efficiency and robustness. Using simulation and DCE-MRI studies in twenty patients with suspected prostate cancer, the VP-based solver was compared against the traditional Levenberg-Marquardt (LM) strategy for accuracy, noise amplification, robustness to converge, and computation time. RESULTS: The simulation demonstrated that VP and LM were both accurate in that the medians closely matched assumed values across typical signal to noise ratio (SNR) levels for both Tofts models. VP and LM showed similar noise sensitivity. Studies using the patient data showed that the VP method reliably converged and matched results from LM with approximate 3× and 2× reductions in computation time for the standard (two-parameter) and extended (three-parameter) Tofts models. While LM failed to converge in 14% of the patient data, VP converged in the ideal 100%. CONCLUSION: The VP-based method for non-linear least squares estimation of perfusion parameters for prostate MRI is equivalent in accuracy and robustness to noise, while being more reliably (100%) convergent and computationally about 3× (TM) and 2× (ETM) faster than the LM-based method.

Technical Aspects of Contrast-enhanced MR Angiography: Current Status and New Applications.

This article is based on a presentation at the meeting of the Japanese Society of Magnetic Resonance in Medicine in September 2016. The purpose is to review the technical developments which have contributed to the current status of contrast-enhanced magnetic resonance angiography (CE-MRA) and to indicate related emerging areas of study. Technical developments include MRI physics-based innovations as well as improvements in MRI engineering. These have collectively addressed not only early issues of timing and venous suppression but more importantly have led to an improvement in spatiotemporal resolution of CE-MRA of more than two orders of magnitude compared to early results. This has allowed CE-MRA to be successfully performed in virtually all vascular territories of the body. Contemporary technical areas of study include improvements in implementation of high rate acceleration, extension of high performance first-pass CE-MRA across multiple imaging stations, expanded use of compressive sensing techniques, integration of Dixon-based fat suppression into CE-MRA sequences, and application of CE-MRA sequences to dynamic-contrast-enhanced perfusion imaging.

Dual echo Dixon imaging with a constrained phase signal model and graph cuts reconstruction.

PURPOSE: The purpose of this work is to derive and demonstrate constrained-phase dual-echo Dixon imaging within a maximum likelihood framework solved with a regularized graph-cuts-guided optimization. THEORY AND METHODS: Dual-echo Dixon reconstruction is fundamentally underdetermined; however, adopting a constrained-phase signal model reduces the number of unknowns and the nonlinear problem can be solved under a maximum likelihood framework. Period shifts in the field map (manifesting as fat/water signal swaps) must also be corrected. Here, a regularized cost function promotes a smooth field map and is solved with a graph-cuts-guided greedy binary optimization. The reconstruction shown here is compared to two other prevalent Dixon reconstructions in experimental phantom and human studies. RESULTS: Reconstructed images of the water and fat signal are shown for a phantom study, and in vivo studies of foot/ankle, pelvis, and CE-MRA of the thighs. The method shown here compared favorably with the other two methods. Large field inhomogeneities on the order of 20 ppm were resolved, thereby avoiding the fat and water signal swaps present in images reconstructed with the other methods. CONCLUSION: Constrained-phase dual-echo Dixon imaging solved with a regularized graph-cuts-guided optimization has been derived and demonstrated to successfully separate water and fat images in the presence of large magnetic field inhomogeneities. Magn Reson Med 78:2203-2215, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Dixon-type and subtraction-type contrast-enhanced magnetic resonance angiography: A theoretical and experimental comparison of SNR and CNR.

PURPOSE: The purpose of this work is to compare the behavior of the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) in contrast-enhanced MR angiography with background suppression performed by either a Dixon-type or subtraction-type method. THEORY AND METHODS: Theoretical expressions for the SNR and CNR for both background suppression techniques were derived. The theoretical Dixon:subtraction SNR and CNR ratios were compared to empirical ratios measured from phantom and in vivo studies for Dixon techniques utilizing one, two, and three echoes. Specifically, the SNR and CNR ratios were compared as the concentration of contrast material in the blood changed. RESULTS: Empirical measurements of the SNR and CNR ratios compared favorably with the ratios predicted by theory. As the contrast concentration was reduced, the SNR advantage of the Dixon techniques increased asymptotically. In the ideal case, the SNR improvement over subtraction contrast-enhanced MR angiography was at least twofold for one- and two-echo Dixon techniques and at least a factor of 6 for the three-echo Dixon technique. CONCLUSION: Expressions showing a contrast concentration-dependent SNR and CNR improvement of at least a factor of two when Dixon-type contrast-enhanced MR angiography is used in place of subtraction-type contrast-enhanced MR angiography were derived and validated with phantom and in vivo experiments. Magn Reson Med 74:81-92, 2015. © 2014 Wiley Periodicals, Inc.

Vascular masking for improved unfolding in 2D SENSE-accelerated 3D contrast-enhanced MR angiography.

PURPOSE: To describe and evaluate the method we refer to as "vascular masking" for improving signal-to-noise ratio (SNR) retention in sensitivity encoding (SENSE)-accelerated contrast-enhanced magnetic resonance angiography (CE-MRA). MATERIALS AND METHODS: Vascular masking is a technique that restricts the SENSE unfolding of an accelerated subtraction angiogram to the voxels within the field of view known to have enhancing signal. This is a more restricted voxel set than that identified with conventional masking, which excludes only voxels in the air around the object. Thus, improved retention of SNR is expected. Evaluation was done in phantom and in vivo studies by comparing SNR and the g-factor in results reconstructed using vascular versus conventional masking. A radiological evaluation was also performed comparing conventional and vascular masking in R = 8 accelerated CE-MRA studies of the thighs (n = 21) and calves (n = 13). RESULTS: Images reconstructed with vascular masking showed a significant reduction in g-factor and improved retention of SNR versus those reconstructed with conventional masking. In the radiological evaluation, vascular masking consistently provided reduced background noise, improved luminal signal smoothness, and better small vessel conspicuity. CONCLUSION: Vascular masking provides improved SNR retention and improved depiction of the vasculature in accelerated, subtraction 3D CE-MRA of the thighs and calves.